Not Applicable
Not Applicable
1. Field of Invention
This invention relates, in general, to optically filtered photodiodes, and more specifically to a photodiode with an integrated microporous optical filter.
2. Description of Prior Art
A photodiode is broadly defined as a device that responds to incident electromagnetic radiation by converting the radiation into electrical energy, thereby enabling measurement of the intensity of the incident radiation. In many applications the incident radiation will not be monochromatic but consist of a spectrum of wavelengths. Often, it is desirable to detect a certain range of wavelengths. In such an application, radiation with wavelengths outside the desired range of wavelengths constitutes noise to the measurement system. Photodiodes with integrated filters were developed to reduce the amount of this noise when measuring a desired wavelength range.
According to Spaeth (U.S. Pat. No. 4,158,133), photodiodes with integrated filters use interference filters for ultraviolet radiation (160 nm to 400 nm photon wavelength). J. F. Seely et. al (Characterization of Silicon Photodiode Detectors with Multilayer Filter Coatings for 17-150 xc3x85; SPIE Vol. 3764. 1999) has demonstrated that thin metallic films such as aluminum or molybdenum can be used as bandpass filters for extreme ultraviolet radiation (1 nm to 160 nm photon wavelength). These filters absorb or reflect a large fraction of unwanted radiation and transmit a fraction of the radiation of interest. Unfortunately, these types of filters can only provide up to five orders of magnitude blocking of the unwanted radiation while transmitting a measurable amount of ultraviolet (UV) or extreme ultraviolet (EUV) radiation. In other words, the amount of noise can only be reduced in magnitude by a factor of no more than about 100,000 using these filters.
Many UV/EUV radiation sources (mainly the sun) produce noise, namely visible light, with magnitude greater than a million times that of the UV/EUV radiation.
According to Gruntman (Submicron structuresxe2x80x94promising filters in EUV. A review. SPIE Vol. 1549 EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy II, p. 385-394, 1991), microporous filtering has been implemented in the micron wavelength region. The reflection of infrared radiation by porous structures with pore diameters in the micron range is utilized in xe2x80x9csuperinsulatorxe2x80x9d shields for thermal protection in high vacuum low-temperature applications. On the other hand, films perforated by channels with much smaller diameter, in the range 10-100 nm, are required for filtering UV/EUV radiation.
Gruntman derives the spectral transmission Tf of a microporous filter as a function of wavelength xcex as follows:
The qualitative dependence of pore transmission can be divided into two regions, transmission for small wavelengths (xcex less than D0) and transmission for large wavelengths (xcex greater than D0). For short wavelengths, the transmission of one hole Tp is asymptotically approaching a value of one as xcex becomes smaller and the total filter transmission approaches G. The quantity G is the geometrical transparency of the filter. For large wavelengths, the transmission drops very steeply with the increase of xcex and the transmission is given by the dependence
Tf(xcex)=Tp(xcex)G where
Tp=D012/(xcex10L2) and
G=Np(xcfx80D02/4)
and D0 is the hole diameter, xcex is the wavelength, Np is the number of pores per filter unit area and L is the microporous filter thickness. For low flux levels, G should be as large as possible and for high flux levels, G should be accordingly low. By varying D0, Np and L, it is possible to tune the filter to pass a given band of wavelengths.
A microporous filter can be formed by using techniques of lithography. X-ray lithography has been used to create regular porous membranes in thin films made from polymers such as mylar as reported by G. N. Kulipanov et. al. (Application of deep X-ray lithography for fabrication of polymer regular membranes with submicron pores, SPIE, Vol. 2723, p. 268-275, 1996). An X-ray mask was fabricated on the base of a boron doped silicon wafer. A titanium and nickel film were evaporated onto the wafer as a background for gold electroplating. To create the holes in the gold absorbing layer, the regular SiO2 columns of 1 micron high and 0.35 to 0.5 micron diameter were originally formed using electron beam lithography. Then, the electroplating of a 1 micron thick gold film was carried out and the SiO2 removed by dry etching. The silicon wafer was then etched from its back side to its etch stops and a silicon membrane of 2 micron thick forms. The mylar film and X-ray mask are then mounted into a vacuum chamber of an X-ray lithography station and exposed to X-rays produced by synchrotron radiation. This process realized 200 nm holes with a very small value of pore diameter spread.
Anodic oxidation of aluminum is yet another method that can be used to form a microporous filter. As reported by R. C. Rumeaux et al. (The formation of controlled-porosity membranes from anodically oxidized aluminum; Nature, Vol. 337, Jan. 12, 1989), the anodic oxidation of aluminum can produce porous films with a remarkably uniform array of cells, each containing a cylindrical pore. The anodizing voltage controls the pore size and pore density, whereas the thickness is determined by the anodization time. Pore sizes of 10 nm to 250 nm, pore densities of 108 g cmxe2x88x922 to 1011 cmxe2x88x922 and film thicknesses of over 100 xcexcm can be achieved.
Accordingly, it is an object of the present invention to provide a semiconductor photodiode with an integrated filter that can provide up to eight orders of magnitude visible light reduction while transmitting a measurable amount of UV/EUV radiation.
A further object of the present invention is to provide a photodiode with integrated microporous filter for UV/EUV radiation detection.
Another object of the present invention is to provide a means of manufacturing a photodiode with an integrated microporous filter which reduces the amount of unwanted radiation (optical noise) by a factor of more than a million.
These and other objects and advantages are provided by a semiconductor photodiode having an integrated microporous optical filter formed over its active surface area. The microporous optical filter is a thin perforated metallic film which serves to transmit radiation with wavelengths shorter than the filter hole diameter and reflect or absorb radiation with wavelengths longer than the filter hole diameter. This type of filter will reduce the transmission of visible light in excess of a million times while still transmitting a measurable percentage of UV/EUV radiation. The microporous filter will be tunable to pass a specified band of UV/EUV wavelengths by selecting the hole diameter, hole pattern, and thickness of the filter. The technology to manufacture 200 nm diameter holes is available at this time and the technology to realize 40 nm diameter holes will be available for production within the next few years.
According to one embodiment, the present invention comprises a single photodiode with integrated microporous filter having predetermined optical properties.
In another embodiment, the present invention comprises a matrix of photodiodes, commonly known as a photodiode array, that are formed in one semiconductor chip with integrated microporous filters.
In another embodiment, the present invention comprises a method of making a photodiode with integrated microporous filter with predetermined optical properties. The method comprises fabrication of a photodiode with an integrated metallic microporous filter by using techniques of photolithography and etching used in the semiconductor industry.
In another embodiment, the present invention comprises a method of making a photodiode with an integrated microporous filter with predetermined optical properties. The method comprises fabrication of a photodiode with an integrated porous alumina filter by using techniques of photolithography in conjunction with anodic oxidation.